a) Field of the Invention
The present invention relates to a zoom lens system which is to be used in still cameras, video cameras and particularly in single-lens reflex cameras, and has a long back focal length, a zooming range covering a standard field angle and a vari-focal ratio of approximately 3.
b) Description of the Prior Art
A zoom lens system which has a zooming range covering the standard field angle and is used in a single-lens reflex camera mostly consists, in order from the object side, of a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, and is configured so as to perform variation of focal length by varying an airspace reserved between these lens units.
As the conventional examples of the zoom lens system consisting of the two lens units as described above, there are known the zoom lens systems disclosed by Japanese patents Kokai Publication No. Sho 51-83,543, Kokai Publication No. Sho 53-91,756, Kokai Publication No. Sho 55-117,118, Kokai Publication No. Sho 56-43,619, Kokai Publication No. Sho 56-132,306, Kokai Publication No. Sho 59-33,417, Kokai Publication No. Sho 59-142,515, Kokai Publication No. Sho 61-80,214 and Kokai Publication No. Sho 63-201,616, Each of these conventional zoom lens systems consists of lens elements in a number as small as seven, but has a vari-focal ratio on the order of 2.
Further, as the conventional zoom lens systems each consisting of lens elements in a larger number and having an enhanced vari-focal ratio, there are known the zoom lens systems disclosed by Japanese patents Kokai Publication No. Sho 53-140,047, Kokai Publication No. Sho 54-72,066,Kokai Publication No. Sho 58-121,011, Kokai Publication No. Sho 60-55,311 and Kokai Publication No. Sho 61-87,117. However, each of these conventional zoom lens systems requires a high manufacturing cost, has a large size and is heavy due to the fact that the zoom lens system adopts a first lens unit which has a large effective diameter and comprises a large number of lens elements, and uses many lens elements which have large outside diameters.
Furthermore, as exemplified by the zoom lens systems disclosed by Japanese patents Kokai Publication No. Sho 57-5,023, Kokai Publication No. Sho 58-132,209, Kokai Publication No. Sho 60-181,717 and Kokai Publication No. Sho 61-170,716, there are known zoom lens systems each of which is configured so as to consist of three lens units by adding a lens unit to the zoom lens system consisting of the two lens units. However, the zoom lens system which consists of the three lens units as described above comprises a large number of lens elements, requires a high manufacturing cost and has a long total length.